# How is wristwatch with 10 years battery life possible?

Turns out Casio offers a handful of wristwatches with "10 year battery life". The claim is that thanks to "an advanced technology" the battery life in those watches is extended to ten years.

Now if you look at different models you see that they are rather complicated hence likely energy consuming - for example, AW-80-1AV model has both a liquid crystal display and hands and also it has LED illumination and a sound alarm.

I first thought that maybe the battery is the key. Model AW-80-1AV runs on CR2025. Energizer CR2025 datasheet specifies that this battery has nominal output voltage of 3 volts and nominal capacity of 163 mAh, so it stores 0,489 volt-ampere-hours of energy.

For comparison, typical basic model of Swatch run about three years on Renata silver oxide 390 (SR1130SW) battery that has nominal output voltage of 1,55 volts and nominal capacity of 60 mAh and so stores 0,093 volt-ampere-hours of energy.

So CR2025 stores about five times more energy, but the basic model of Swatch only has hands - no digital display, no illumination, no alarm, so it likely consumes less energy.

There clearly must be something more than a bigger battery that makes 10 years battery life possible.

How is 10 years battery life possible in a rather energy consuming wristwatch?

-

10 years =~ 87650 hours.
1 uA drain will require 87.75 mAh in 10 years.
With som shelf life degradation that's close enough to
= 10 mAh / uA / year or
= 100 mAh / uA / 10 years

So your cited 163 mAh battery will supply 1.63 uA mean.
Pushing technology, size and luck may get you to say 5 uA mean.

There are 86400 seconds/day. There are 1440 minutes/day.

You will find that eg alarm use is much restricted in the allowable use to get 10 years. If 1 uA of the drain is for alarm use then you get 24 uA.hr/day or 86400 uA.seconds or 86 mA.seconds. That's about 240 mW seconds at 3 V. Or say 5 x 50 mW x 1 second burst/day.

An LED can provide ample lighting at 1 mA. Use it 5 times/day x 1 second = 5 mA.sec = 5000 uA.sec or "only" 5000/86400 = 0.06 uA mean drain. Increase as desired and allowed.

Can you run a time keeping IC on say 1 uA?
Probably yes.

So overall it all falls in the area of "notionally possible if really really really clever and careful".
Casio can be expected to be quite clever by now.

Note that if any sort of energy harvesting is being used then all bets are on. Harvesting a uA or few sounds doable.

-
It's not only timekeeping that works continuously - there's also a display and hands that work 24/7/365. –  sharptooth Sep 18 '12 at 8:16
Energy harvesting sounds likely. LCD can be lowish power. Hands are annoying. If they step at 1/second and if we assign them 1 uA then they get 1 uA.sec.sec = not much. –  Russell McMahon Sep 18 '12 at 8:19
An EFM32, which is an ARM Cortex M3 MCU, can run on about 1.45µA while driving a LCD (550nA for the LCD, and 900nA for running the RTC and keeping its RAM). So a chip keeping only time should be capable to run on much less than that. –  hli Sep 18 '12 at 9:32
I inherited my late grandfather's Boluva. It works fine as long as it gets moved periodically, e.g. I wear it. No alarm, but hands and a calendar. Possibly some energy for the Casio watch could be supplied in a similar manner to reduce the load on the battery. –  rdivilbiss Sep 19 '12 at 0:36
@RussellMcMahon: I agree that the hands probably use most of the power. The datasheet seems to indicate the Model AW-80-1AV only moves its hands once every 20 seconds. I don't see how that saves power, though -- doesn't it use the same amount of energy to move a hand a little at a time as to move the same distance all at once? –  davidcary Sep 19 '12 at 15:36

Perhaps those watches use some kind of energy harvesting system to recharge a rechargeable battery?

An automatic quartz watch has an energy harvesting mechanism that, like a mechanical self-winding watch, pulls small amounts of energy from the day-to-day motions of the person wearing it.

A solar-powered watch uses a tiny solar cell to pull energy from ambient light. (Even indoor light, much dimmer than sunlight, is adequate to keep the watch running).

I hear that such watches typically run normally for a day or so when cut off from outside power (taken off the wrist, put in a dark room, etc.). Then they enter low-power state where everything is turned off except internal timing -- the LCD display goes blank, the hands stop. The watch has a power reserve that can keep internal timing going for at least a month; one manufacturer claims it has watches with a 4 year power reserve. Then when you pick it up and shake it, the battery starts to charge up, and the "hands will magically spin to match the current time." ( a ).

Are you looking for detailed information on how it's possible to build electronic devices with extremely low power consumption? Then you might enjoy reading Jeelabs's notes on low power electronics ( b ). One JeeNode has run for over 2 years on a single battery charge (charged on August 21st, 2010; still running and continuing to count on Sep 15, 2012). ( c )

Or are you looking for techniques to keep a battery from failing prematurely? While recharging a battery many times over 10 years is "easier" than trying to get a primary cell to last 10 years, it doesn't make it "easy". I've bought new rechargeable batteries for several consumer electronics devices when the original rechargeable battery went dead in less than 5 years -- not merely drained, but completely dead. (What makes this especially frustrating is when 5-year-old equipment uses batteries that are some special shape that stopped production years ago and are now unavailable, and I suspect all the shiny new oddly-shaped batteries will likewise be unavailable in 5 years.). How to keep batteries from failing prematurely would make a good separate question -- I wish I knew the answer.

-
If the charger is not optimised for battery lifetime, then taking the battery off charge once charged and not allowing it to deep discharge will together increase lifetime. LiIon batteries have a calendar life regardless if use. NiCd and NimH do not. LiFePO4 may not (not much comment available thereon). –  Russell McMahon Sep 19 '12 at 17:02